Just as a heads-up, TLS 1.3 is now exposed in the Privacy & Security > SSL/TLS preferences pane on trunk nightly builds and will be allowed to ride the release train. While there is still ongoing work on the NSS backend, it has been made available in the UI for testing purposes. By default, the maximum protocol remains TLS 1.2, thus you have to explicitly enable it. Websites supporting TLS 1.2 or lower only shouldn't be affected even if TLS 1.3 is enabled, as long as the respective TLS variants are allowed in the preferences.